Wireless communication systems are prevalent in today's world and are relied upon by practically everyone for day-to-day communications. Wireless communication systems typically are in the form of cellular networks or mobile networks which are wireless communication networks distributed over land areas called cells. Each cell is generally served by at least one fixed-location transceiver which is known as a cell site or the base station (“BS”) of the cell. The base station communicates with various user equipment (“UE”) types, such as cellular telephones, pagers, wireless notepads, computers and other mobile communication devices.
In wireless communication systems, time and frequency synchronization is essential for the communication of data. Typically, a receiver uses one or more signals transmitted by a transmitter to obtain and maintain synchronization. One example is the downlink (DL) in a wireless communication system, where a base station is a transmitter and a user equipment is a receiver. In order to advantageously and successfully receive a data transmission from a BS, the UE needs to be synchronized to the BS transmitter. The same is true for uplink (UL) transmission of data from a UE to a BS.
One aspect of synchronization is time synchronization. Time synchronization can mean that a UE detects the temporal structure of the signal received from the BS, on one or more levels of granularity. In LTE (Long Term Evolution) networks, for example, a UE can detect the symbol, slot, subframe, radio frame timing, etc., in the signal received from an evolved nodeB (“eNodeB,” an LTE BS) or other BS. The time synchronization can be used to correctly extract other parts of the signal in time, for example parts carrying information and data.
Another aspect of synchronization is frequency synchronization. Frequency synchronization can mean that a UE estimates the carrier (center) frequency of the signal received from a BS in some examples but other frequency synchronization techniques are also used. Frequency synchronization can be used to improve the data reception performance.
As used herein, the term synchronization is used to refer to time synchronization, frequency synchronization or both.
In many cellular systems, the cells are distinguished from one another, from the perspective of the UE, by cell identity numbers (cell-ids). The cell-id is often reflected in different transmission characteristics for different cell-ids such as, for example, reference signal sequences, scrambling sequences, synchronization signal properties, and the like. A UE may able to detect the identity of the cell it is in, from certain estimated physical layer parameters, for example, and subsequently learn other properties such as physical layer properties from the detected cell-id. In some cellular systems, a BS can serve multiple cells, i.e. transmit and receive signals corresponding to multiple cells.
LTE (Long Term Evolution) and LTE-Advanced systems are cellular systems hereinafter referred to collectively as LTE cellular systems, LTE systems, LTE networks or simply LTE. UEs in LTE networks can detect the LTE network by searching for the primary and secondary synchronization signals (PSS and SSS), which are periodically transmitted by an evolved node B (“eNodeB” or “eNB”). From the detection of the primary and secondary synchronization signals, the UE can also learn the cell identity number (cell-id), which in LTE may be called physical cell identity (PCI). From the PCI, the UE learns the time and frequency location and the sequence used for the cell specific reference signal (CRS).
A UE is typically connected or associated with a cell which may be called the UE's serving cell. A cell from which a UE receives data and/or to which a UE transmits data is typically referred to as the serving cell. In other words, the UE functions by communicating with, exchanging information with, or carrying out actions with, its serving cell. In some examples, a UE is connected or associated with more than one cell, so that it can receive data from and/or transmit data to multiple cells. In such examples, the UE has multiple serving cells. This is a scenario if a UE is capable of carrier aggregation or coordinated multipoint (CoMP), such as in LTE Advanced networks.
Many of the actions a UE performs are directed towards a certain cell. Such a UE action is referred to herein as a cell-directed UE action. Examples of cell-directed UE actions include but are not limited to the following: A UE performs a measurement on signals from a certain cell. A UE initiates a random access procedure in order to connect to a certain cell, for example as part of a handover. A UE receives from or transmits to a certain cell.
These examples are discussed in further detail below. In each of these examples, the UE typically performs these actions in a way that is synchronized to the cell currently serving the UE. Different examples may require different levels of synchronization accuracy.
UEs are mobile devices that are capable of movement throughout geographical areas and the UE needs to be able to function with an associated cell as it moves about. In order to facilitate UE mobility in cellular systems, one cell-directed UE action involves a UE performing measurements on signals from different cells. For example, the received signal power of a reference signal can be measured. In LTE and other systems, a UE can measure and report reference signal received power (RSRP) or reference signal received quality (RSRQ). In order to perform a measurement on a signal from a cell, a UE is typically synchronized to the cell. In LTE, for example, the cell specific reference signal (CRS) is transmitted in a subset of the subframes and on a subset of the symbols in a subframe. Therefore, in order to perform a measurement on the CRS from a particular cell, the UE can extract the time-frequency parts of the received signal that contain the CRS from the cell it desires to measure, using time and frequency synchronization to that cell.
Another cell-directed UE action involves the UE initiating a random access procedure in order to connect to a cell. This can be advantageous if the UE has been idle for a long time, or is performing a handover between cells. A random access procedure typically starts with the transmission of a first random access signal by the UE. This signal can be transmitted in accordance with time and frequency synchronization of the cell to which the UE wishes to connect. In LTE, for example, the first random access signal is called the physical random access channel (PRACH) preamble. The PRACH may be advantageously transmitted in well-defined time-frequency resources of the cell in order to be detected by the eNB.
In another cell-directed UE action, a UE receives data from a cell or transmits data to a cell. In order to do this, a UE needs to be synchronized to the cell in conventional systems. In an LTE cell for example, the downlink and uplink transmissions may be confined to a set of resource blocks, which have well-defined time and frequency parameter values, in relation to the synchronization of the cell.
Each of the aforementioned cell-directed UE actions, i.e. UE measurements, random access, and data reception and transmission, involves the UE being synchronized with the cell.
A cell towards which a UE performs a cell-directed UE action is called a target cell. Hence, consistent with the previously described examples, a UE can perform a measurement on a signal from a target cell; a UE can initiate random access towards a target cell, for example by transmitting a random access signal; a UE can receive a data transmission from a target cell; and a UE can transmit data to a target cell. Further, consistent with the previous description, in performing these cell-directed UE actions, the UE becomes synchronized to the target cell.
Synchronization to the target cell may be unsatisfactory with respect to accuracy, power consumption, complexity, time duration, or with respect to other issues between the UE and targeted cell, however.